Mobile communication is one of the most important technologies for contributing to social and economic development around the world. Optimizing energy efficiency will not only reduce environmental impact, it will also cut network costs which will give benefits for all using the mobile systems.
Capital expenditure typically represents a very small portion of the total cost of the ownership. Instead, the long term savings from site reduction and efficient operation is significant, with a significant reduction in energy consumption as a key issue.
Optimizing solutions for reducing energy consumption means that every stone has to be turned over. Still, the total network solution is greater than the sum of their parts. This means that combining the best components in a package does not always give the best results. In the radio base station the relative energy consumption of the different components vary on the dependency of the properties of the components it has to work with.
Typical sources of energy consumption in the base station are signal processing, RF conversion, power amplification, power supply, climate equipment (air conditioning) and feeder. For instance, In traditional base stations the equipment is located on the ground which means that the antennas has to be fed using several meters of cable. Half of the emitted power can be lost in the feeders. By placing the equipment in the top of the tower, significant reductions in energy consumption is achieved. The equipment can be combined with a battery back-up unit that minimizes hardware and energy consumption.
One way of reducing the energy consumption is to avoid unnecessary DC/DC conversion and reduce the need of cooling fans and cooling systems. Modules based on digital power management can also reduce energy consumption.
Another way in which energy reduction can be achieved is through the use of stand-by modes. Base station sites are dimensioned to cope with peak hours. In a cell a number of TRX (transmitters) can run at the same time. Using power management schemes, some TRX can be put in stand-by instead of running in idling mode during low traffic hours.
Network design is a key issue improving the energy efficiency. In order to achieve an energy-efficient design a number of issues have to be addressed from start. At first, the true network needs has to be addressed. No amount of energy efficiency at the component level can make up for an inefficiently deigned network. For instance the number of radio base stations should be optimized for the coverage and quality that needs to be achieved.
The exact coverage, capacity and quality have to be considered before getting into considerations about individual sites and equipment specifications. Moreover, the current and future business environment needs has to be considered, considering the possibility to rebuild or expand sites. Once these factors have been considered the operator should begin the network design process, looking into the total cost of the ownership and the alternative design options.
The radio base stations use a large amount of energy. The main task of the base transceiver station is to enable communication with the user's terminal being positioned in the cell. The cells are served by radio transceivers arranged in a base station. These cells are used to cover different areas in order to provide radio coverage over a wider area that the area of one cell.
FIG. 1 shows the cellular structure in a telecommunication radio network. In the lower part of FIG. 1 is a separate cell with a base station disclosed for clarity reasons. The cells 10,11 overlap 12 to avoid areas without coverage. There are various solutions for distinguish the signals from different transmitters in the different base stations, such a Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA). Handover is used between the cells for moving user terminals.
The smaller cells 10 are used within city centres where there are a lot of buildings which shields the signal and where there are a lot of users. With smaller cells more channels are available in a certain geographical area which enables communication with more users. The smaller cells provide radio coverage and additional capacity where there are high numbers of users. The base station 13 antennas for these smaller cells are mounted at street level, typically on the external walls of existing structures, lamp posts and other street furniture. Typically, these cells provide radio coverage across smaller distances and are placed 300 m-1000 m apart. They have an output in the range of a few watts.
The larger cells 11 provide the main coverage in a mobile network. The antennas for the larger cells are mounted on ground-based masts, rooftops and other existing structures. They must be positioned at a height that is not obstructed by surrounding buildings and terrain. Large cells base stations have a typical power output of tens of watts. These cells also cover the countryside where there are less obstacle and less users per area unit.
Since the cell covers a geographical area, the transmitting power of the transceivers is a function of the coverage of the transmitted signals enabling the communication. An obstacle in the area which shields the transmitted signals means that the power may have to be increased even more to provide the quality of service that the operator aims for.
In addition to the expanding telecommunication network requiring more and more base stations, there is an increasing need of delivering wireless technology with broadband capacity for cellular networks. A good broadband system must fulfil certain criteria, such as high data rate and capacity, low cost per bit, good Quality of Service and greater coverage. High Speed Packet Access (HSPA) and Mobile WiMAX are examples two network access technologies that enable this.
These modern standards have very high capacity in terms of users and throughput, which requires a large amount of energy. In order to achieve high data throughput in the cellular systems a dense cell plan has to be deployed. A base station for modern standards consumes considerable amount of power, typical 65000 kWh per station and year.
For instance mobile WiMAX transmitted via base stations uses Scalable Orthogonal Frequency Division Multiple Access (SOFDMA) with Time Division Duplex (TDD). FIG. 2 shows a schematic view of a frame structure for OFDMA (on which SOFDMA is based) when operating in TDD mode. Some WiMAX systems support OFDMA operating in Frequency division duplexing (FDD) in which the frame structure differs from TDD in that the uplink and downlink frames are transmitted at the same time over different carriers. The frame (Frame N) comprises a downlink subframe 15, a following uplink subframe 16, a small guard interval 20 between the downlink and uplink subframe and an end interval 22 between the uplink and the downlink subframe of the next frame.
The downlink subframe 15 in TDD begins with overhead information for informing the user device about the characteristics of the system. The overhead comprises synchronization information 17 or system information 18. The overhead is followed by data regions 19 for the downlink data traffic in the downlink subframe. A guard interval 20 is followed by an uplink subframe 21 with data regions for the uplink data traffic from the different user devices. Finally there is the end interval 22 followed by the overhead synchronization information 17 of the next frame.
In WiMAX particularly the overhead begins with a downlink preamble that is used for physical-layer procedures (cell detection, time and frequency synchronization). The preamble is followed by a frame control header providing frame configuration and system information (modulation and coding maps) to find where and how to decode downlink and uplink. The frame control header and maps are sent for each available data region 19, 21.
In order to achieve high data throughput in cellular systems, high order modulation, e.g. 64 QAM and high transmit power is used at the base station. The physical resources in term of subcarriers and time are kept to a minimum to maximize the user data throughput. High performance power amplifier is needed to keep the signal properties after the amplification. Especially the linearity of the amplification is important. This requires a lot of energy which increases the energy consumption of the base station. Due to these requirements the amplifier efficiency is low and contributes to a large extent the base station power consumption.
During low load or no load scenarios the base station still needs to transmit the system and synchronization information 17,18 to serve the attached user terminals and so a new terminal can access the system. The information has to be transmitted with enough power to reach all user terminals within the cell and is therefore transmitted with low modulation order and high output power. Due to these transmissions the base station power consumption is still quite significant.